If you perform an operation on a table in writer, such as adding or deleting a row or column, you can Undo it, but you can’t Redo it. Why is that? (Word allows you to redo such operations…)
Also Writer allows to do such operations. I just made a quick test in LibO 4.2.6.3 on an XP machine and it work fine.
Test was adding and deleting rows and columns and ctrl+z undid what I did before.
Additionally in my version I cannot set the number of Undos to 0. Thus at least 1 undo is always possible. You can check the settings:
Tools > Options > LibreOffice > Memory > Undo and select the number of steps.
May I ask you to please report a bug with detail description on what you did and what the result of each step was? Please file the bug report here: https://www.libreoffice.org/get-help/bug/